serial: Tidy REMOTE_DEBUG

REMOTE_DEBUG does already appear in 2.2 kernel sources but didn't
appear as a config Option in the initial git import 2.6.12-rc. It's
currently just used in one single place of the linux kernel and should
probably be dropped totally

Signed-off-by: Christoph Egger <siccegge@cs.fau.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>